Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Complexed Starch-containing compositions having high mecha-
nical properties
The present invention relates to heterophasic polymeric
compositions having a high resistance to ageing, even under
conditions of low temperature and humidity, comprising
thermoplastic starch and a thermoplastic polymer
incompatible with starch, in which the starch constitutes the
dispersed phase and the polymer the continuous phase.
The invention relates particularly to manufactured products
which maintain high impact strength and tear strength in low
humidity conditions.
It is known that products (in particular films) manufactured
from compositions containing thermoplastic starch and a
thermoplastic polymer incompatible with starch, in which the
starch constitutes the dispersed phase, show a significant
dete,rioration in their mechanical properties, in particular,
their impact strength and tear strength, due to the fact that
the starch gives up or absorbs water until it reaches
equilibrium with the ambient humidity at its interface.
In relatively low humidity conditions, the material tends to
become brittle, as the dispersed phase becomes insufficiently
plasticised due to the loss of water which takes the glass
transition temperature above ambient temperature.
This phenomenon can damage the interface with the matrix when
the interface is not sufficiently bonded.
Under these conditions, when the starch particles
constituting the dispersed phase are subjected to stress,
they are unable to deform and absorb the stress, but instead
remain rigid, thus initiating a tear.
Italian patent application No. T096A000890 filed by the
CONFIRMATlON C PY
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Applicant describes compositions comprising thermoplastic
starch and a thermoplastic polymer incompatible with the
starch, having improved characteristics of resistance to
ageing under conditions of relatively low humidity, obtained
by introducing an agent having an interfacing action during
.the mixing of the components. This compatibility-inducing
action improves the adhesion between the matrix and the
dispersed particles.
Reducing the interface tension also enables the dimensions of
the particles to be reduced to submicronic values, whereby
the materials have the characteristics of a polymeric alloy.
Compositions comprising starch , a thermoplastic polymer and
a plasticiser are widely described in patent literature.
However, the concentrations of these plasticisers at which
the mechanical properties of the compositions are greatest
are never tau,ght, nor suggested, in the prior art.
EP-A-0 327 505 describes compositions in which the
plasticiser is used in a quantity of from 0.5 to 15%,
preferably between 0.5 and 5% by weight, together with such
quantities of water that the sum of the plasticiser and the
water does not exceed 25% by weight of the compositions (the
quantitative minimum of water in these compositions is 10%
by weight).
W092/19680 describes compositions comprising starch, a
polyester of a hydroxyacid or the corresponding lactone such
as, for example, polycaprolactone, and a plasticiser usable
in a quantity of from 1 to 50% by weight, preferably 1-40%,
and more preferably 5-25% by weight of the composition.
The compositions preferably have a final water content of
between 1.5 and 5% by weight (measured on leaving the
extrusion press, before conditioning).
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In the aforementioned document, there is no use of nor any
indication of the existence of a possible critical range of
the concentration of the plasticiser corresponding to that
for obtaining very high mechanical properties, nor is there
any indication of which plasticisers are suitable for this
purpose.
The quantity of plasticiser used in the examples is always
greater than 10% by weight of the composition.
US-5 334 634 describes compositions comprising starch, an
ethylene-vinyl alcohol copolymer and a plasticiser usable in
a quantity of from 0.5 to 100% by weight of the starch .
In this case also, the quantity of plasticiser effectively
used is always greater than 10% by weight of the composition.
It is known that starch, in particular, its amylose fraction,
forms "V"-type complexes with synthetic polymers such as
polyethylene vinyl alcohol or polyethylene-acid acrylate
(C.Bastioli and others in "Biodegradable Plastics and
Polymers", pages 200-213; 1994, Elsevier Science). In such
multiphase systems in which the synthetic polymer comprises
the continuous phase and the starch the dispersed phase, the
complex acts as a compatibility-inducer or phasing agent.
Similar complexes can form between starch and aliphatic
polyesters or aliphatic/aromatic copolyesters. However, if,
in the preparation of the compositions comprising starch
and the aforementioned polyesters, relatively high quantities
of the starch plasticisers are used to ensure the plasticity
of the material under the conditions of use of the
manufactured product and low specific energy for
destructurization and complexation is used, the quality of
the interface is insufficient to ensure the toughness of the
material at low temperatures and humidity in the presence of
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the plasticiser itself.
Furthermore, if plasticisers which are solid at room
temperature are used in relatively high concentrations, at
which the complex between starch and incompatible polymer
can form in a quantity sufficient to ensure an effective
compatibility-inducing action, these plasticisers cause, in
conditions changing from high to low relative humidity,
brittleness in the material.
It has unexpectedly been found that it is possible to prepare
heterophasic compositions comprising starch and a
thermoplastic polymer incompatible with starch, in which the
starch constitutes the dispersed phase and the thermoplastic
polymer the continuous matrix, which compositions have
characteristics of high impact strength even when passing
from conditions of high to low relative humidity if they are
prepared using a quantity of plasticiser that is liquid at
room~ temperature comprised within a critical range wherein
the concentration of the complex between starch and the
incompatible polymer reaches a maximum, and a specific energy
of destructurization of starch higher than a certain value.
The critical quantity of plasticiser, which is preferably
glycerin, is generally from 2 to 8% and preferably from 3 to
7% by weight of the starch and the thermoplastic polymer.
Quantities outside this range are, however, possible,
depending on the type of plasticiser and its efficacy.
The specific energy for the destructurization of the starch
and its complexation are comprised from 0,1 to 0.5 Kw.h/Kg,
preferably from 0,15 to 0,4 Kw.h/Kg and most preferably from
0,2 to 0,35 Kw.h/Kg.
For specific energy for the destructurization and
complexation of the starch it is to meant the energy supplied
by an extruder the screw or screws of which are capable of
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developing.a specific energy of at least 0,1 Kw.h/Kg at the
extrusion temperature of 120 - 210 C.
The specific energy is determined according to the formula: A
x B x C / DxExFwherein
A = engine power
B= RPM
C= energy absorption
D= RPM max
E= energy absorption max
F= flow rate
Until now, critical values as indicated above had never been
used nor suggested in prior art compositions.
It has been discovered, and this constitutes a characterising
aspect of the invention, that the complex of starch and
incompatible polymer reaches maximum concentration values
within the aforesaid critical range.
The presence of the complexes of starch and incompatible
polymer can be demonstrated by the presence in the second
derivative FTIR spectra of a band at a wavelength of 947 cm-1
(specific to the complex) and in the X-ray diffraction
spectra of a peak in the range of 13-14 on the 2 theta
scale (with Cu Kalfa radiation of 1.5418 A ). In both cases,
the position of the band or the peak of the complex remains
unchanged, even on changing the nature of the complexed
polymer. Figures 1 and 2 show the X-ray and second derivative
FTIR spectra, and are typical of the formulations based on
starch and aliphatic polyesters (PCL in particular).
It has been found that in the X-ray spectra of the
compositions of the invention, the Hc/Ha ratio between the
height of the peak (Hc) in the range of 13 - 14 of the
complex and the height of the peak (Ha) of the amorphous
starch which appears at about 20.5 (the profile of the peak
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in the amorphous phase having been reconstructed) is less
than 2 and greater than 0.02. In the spectrum of Figure 1,
the heights Hc and Ha, are indicated for the peaks of the
complex and the amorphous starch respectively.
In case of crystalline polymers with a crystallinity content
higher than 30% the lower limit of the ratio Hc/Ha is 0.2; in
case of amorphous polymers or polymers with a cristallinity
content less than 30% the lower limit of the ratio Hc/Ha is
lower than 0.2.
The heterophasic compositions of the invention therefore
comprise starch, a thermoplastic polymer incompatible with
the starch, a starch plasticiser or a mixture of starch
plasticisers, in which the starch constitutes the
discontinuous phase and the thermoplastic polymer the
continuous phase, and are characterised in that they form
films having characteristics of high impact strength higher
than,, 30 Kj/m2, preferably higher than 45 Kj/m2 and most
preferably higher than 60 Kj/m2 (measured on blown film 30
micron thick at 10 C and less than 5% relative humidity) and
have an X-ray spectrum having a peak at angle 2 theta in the
range from 13 to 14 with an intensity related to that of the
peak of the amorphous starch which appears at an angle 2
theta of 20.5 less than 2 and greater than 0.02.
The compositions are obtainable by extrusion of a melt
comprising starch, the thermoplastic polymer, the plasticiser
in a quantity within the critical range, and water in a
quantity less than 5% by weight (measured on leaving the
extrusion press, before conditioning) and supplying a
specific energy of at least 0,1 Kw.h/Kg and lower than 0,5
Kw.h/Kg.
The preparation of the compositions by extrusion is carried
out according to known temperature conditions, operating, for
example, at temperatures of between 120 and 210 C, preferably
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from 130 to 190 C.. Suitable usable extruders are those
provided with screws having a "reverse" profile for more than
30% of the length of the screw (a reverse profile causes the
material to advance with a piston effect).
The water content in the extrusion stage can be high in the
phase of destructurization of starch and can be regulated at
the end of the estrusion at the desired values of less than
5% by weight by degassing or by using a starting starch with
a low water content (the water content is measured at the
exit of the extruder, prior conditioning).
If the compositions or the manufactured products obtainable
therefrom are washed with water, the plasticiser contained
therein is extracted but the compositions and the
manufactured product maintain mechanical properties, in
particular impact strength, comparable to the properties of
the film before washing. These compositions and manufactured
products also,form part of the invention.
The starch-incompatible thermoplastic polymers are preferably
chosen from the aliphatic (co)polyesters obtained from
hydroxyacids having 2 or more carbon atoms, and from the
corresponding lactones or lactides, or from aliphatic
bicarboxylic acids.having 2-22 carbon atoms, and from diols
having 2-22 carbon atoms, polyester-amides, polyester-urea
and aliphatic-aromatic copolyesters and mixtures thereof.
These thermoplastic polymers, or mixtures thereof, have a
melting point lower than 130 C and preferably lower than
110 C.
Representative examples of the polymers mentioned above are:
- poly-epsylon-caprolactone, polyethylene- and polybutylene-
succinate, polyhydroxybutyrate-hydroxyvalerate, polylactic
acid, polyalkyleneadipate, polyalkyleneadipate-succinate,
polyalkyleneadipate-caprolactame, polyalkyleneadipate-epsylon
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-caprolactone, polyadipate of diphenol diglycidylether, poly-
epsylon-caprolactone/epsylon-caprolactame, polybutylene adi-
pate-co-terephthalate, polyalkylenesebacate, polyalkylene-
azelate and copolymers thereof or mixtures thereof.
These polymers can also be "chain-extended" with
diisocyanates, polyepoxides and similar multifunctional
compositions.
Poly-epsylon-caprolactone and the aliphatic-aromatic
copolyesters are preferred. Other polymers which can be used
are the esters and ethers of cellulose and of starch.
The starch-incompatible polymer is present in a quantity
sufficient to form the continuous phase of the heterophasic
composition. In general, this quantity is between
approximately 30 and 90% by weight of the starch.
The ,,polymers can be used in mixtures having smaller
proportions of polymers of the ethylene/vinyl alcohol,
ethylene/acrylic acid type and polyvinylalchol.
The usable starch is native starch such as, for example,
corn, potato, rice, tapioca starch, or is a physically or
chemically modified starch such as, for example, ethoxylated
starch, starch acetate and hydroxypropylated starch, cross-
linked starch or oxidated starch, dextrinized starch,
dextrins and mixtures thereof.
The starch plasticisers which can be used are polyhydric
alcohols having from 2 to 22 carbon atoms, in particular,
polyhydric alcohols having from 1 to 20 hydroxylated units
containing from 2 to 6 carbon atoms, the ethers, thioethers
and the organic and inorganic esters of these polyhydric
alcohols.
Examples of plasticisers that can be used are: glycerine,
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ethoxylated. polyglycerol, ethylene glycol, polyethylene
glycol, 1,2-propandiol, 1,3-propandiol, 1,4-butandiol,
neopentylglycol, sorbitol monoacetate, sorbitol diacetate,
sorbitol monoethoxylate, sorbitol diethoxylate and mixtures
thereof.
The compositions can also include interfacial agents of the
kind described in Italian patent application T096A000890,
chosen from:
a) esters of polyhydric alcohols with mono- or
polycarboxylic acid having a dissociation constant pK less
than 4.5 (with reference to the pK of the first carboxylic
group in the case of the polycarboxylic acids), and a
hydrophilic/lipophilic index (HLB) greater than 8;
b) esters of polyhydric alcohols with mono- or
polycarboxylic acid having fewer than 12 carbon atoms, pK
values greater than 4.5, and HLB indexes of from 5.5 to 8;
c) esters of polyhydric alcohols with C12-C22 fatty acids,
having an HLB: index of less than 5.5;
d) non-ionic, water soluble surfactants, and
e) products of the reaction between aliphatic or aromatic
diisocyanates and polymers containing terminal groups that
react with the diisocyanates.
The compositions of the invention can also contain additives
such as urea in a quantity of up to 20% by weight, compounds
of boron, particularly boric acid, proteins such as casein,
gluten and abietinic acid or rosinic acid, natural rubbers,
flame retardant agents, antioxidants, fungicides, herbicides,
fertilisers, opacifiers, compositions having a repellent
effect on rodents, waxes, antislipping agents (such as
erucamide, calcium stearate, zinc stearate).
They can also contain organic and inorganic fillers from 0.5
to 70% by weight and natural fibers. The compositions of
the invention find particular application in the preparation
of films, sheets, in thermoforming and, in general, in all
CA 02334336 2004-09-21
applications in which good mechanical properties of the manufactured
product are required, together with high resistance to ageing, even
under conditions of low temperature and humidity.
Examples of products which can be manufactured using the compositions
of the invention include, in addition to those mentioned above, bags,
laminates, moulded and blown articles, expanded sheets, expanded
materials, biofillers for tyres, backsheets for diapers, wrapping
films, mulching films, multilayer films, sacks for mowing grass,
shoppers, nonwoven fabric, toys, pet toys, dog collars, products with
controlled release for use in the agricultural field, threads.
The following examples are given to illustrate and not to limit the
scope of the invention.
Example 1
A mixture formed from (parts by weight):
- Globe 03401-Cerestar'" natural starch* 27
- Tone-787TM PCL 65
- glycerine 4.5
- water 3.5
100.0
* water content 12% wt
was supplied to a two screw OMC extruder of 60 mm diameter, L/D = 36,
RPM = 180.
The temperature profile was as follows: 60/145/175/180x4/155x2 C.
It was operated with free degassing.
The specific energy supplied was 0,4 Kw.h/Kg.
The extruded material was pelletised. The water content was 1.3% by
weight.
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The pellets were used to manufacture films using GhioldiTM apparatus
provided with Maillefer"-type screws of 60 mm diameter and L/D = 30.
The thermal profile was as follows: 90/120/140/150x3/147x2 C.
The film head had a diameter of 180 mm.
The film produced, approximately 30 thick, was tested as such for its
mechanical properties. A sample of the same film was on the other hand
immersed in water for 24 hours to remove the starch plasticisers; after
this, the samples taken from the washed film were left to condition for
72 hours in an environment with a temperature and humidity equal to
those used for detecting the mechanical properties.
Example 2
A mixture of (parts by weight):
- Globe 03401 corn starch 33.4
- Tone-787 PCL 54.3
- glycerine 5.8
- water 6.5
100.0
was supplied to a two screw APV-2030T'" extruder; L/D = 35 + 5XLT;
RPM = 170; thermal profile: 60/100/170x14 C.
The extruder was operated with free degassing.
The specific energy supplied was 0,17 Kw.h/Kg.
The extruded material was pelletised. The water content was 1.5% in
weight.
The pellets were used to produce a sheet via cast-extrusion, using a
modified AEMME'"' extruder provided with 1:3 constant taper screws;
diameter 30 mm; L/D = 25; RPM = 35. The extruder had a flat head 150
mm wide with a lip aperture of 0.8 mm. The sheet obtained was 0.6 mm
thick.
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A quantity of pellets was separately made into a film as described in
example 1 to obtain samples to test for their mechanical properties
(samples of the film as produced and washed in water).
Example 3
The test of example 2 was repeated using a composition (parts by
weight) of 33.4 parts starch of the type used in example 2, 54.3 parts
of Tone-787 PCL, 4.8 parts of glycerine and 7.5 parts of water. The
film thus obtained was tested for its mechanical properties (film as
produced and washed in water).
Comparison Example 1
A mixture formed from (parts by weight):
- Globe 03401 corn starch 33.4
- Tome-787 PCL 54.3
- glycerine 9.7
- water 5.5
100.0
was mixed in an extruder and made into a film as in example 1.
The specific energy supplied was 0,22 Kw.h/Kg.
The film obtained was tested for its mechanical properties (film as
produced and washed in water as in example 1).
Comparison Example 2
A composition comprising 65 parts potato starch at 6% humidity and 35
parts of a mixture of glycerine: sorbitol 1:1 by weight (sorbitol is
solid at ambient temperature) was supplied to the two screw APV-2030
extruder, as used in
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example 1,.operating with the following thermal profile:
60/100/190x14 C. Compounding was done with active degassing
to obtain an extrudate having a water content of less than
0.5%.
Then, 35 parts of dried pellets and 65 parts of Tone-787 PCL
were mixed in an APV-2030 extruder; the extruded material was
made into pellets and finally made into a film of
approximately 30 p thickness, exactly as in example 1.
Example 4
The test of example 3 was repeated with the only difference
being that 3.8 parts glycerine and 8.5 parts water were used.
The film thus obtained was tested for its mechanical
properties (film as produced and washed in water as in
example 1).
Example 5
A mixture formed from (parts by weight):
- Globe 03401-Cerestar natural starch 26.4
- Ecoflex 63.8
- glycerine 5.5
- water 4.3
- Erucamide 0.3
100.0
was supplied to a two screw OMC extruder of 60mm diameter,
L/D = 36, RPM = 180.
Ecoflex is a registered trade mark of BASF and refers to a
polybutylene adipate-co-terephthalate copolymer.
The temperature profile was as follows: 60/140/175/180x4 C.
It was operated with free degassing.
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The specific energy supplied was 0,36 Kw.h/Kg.
The extruded material was pelletised. The water content was
1.7% by weight.
The pellets were used to manufacture films using Ghioldi
apparatus provided with Maillefer-type screws of 60mm
diameter and L/D = 30. The thermal profile was as follows:
120/135/145x5/140 C.
The film head had a diameter of 100mm.
The film produced, approximately 30 u thick, was tested as
such for its mechanical properties. A sample of the same
film was on the other hand immersed in water for 24 hours to
remove the starch plasticisers; after this, the samples
taken from the washed film were left to condition for 72
hours in an environment with a temperature and humidity equal
to those used for detecting the mechanical properties.
TABLE 1
PROPERTIES OF BLOWN FILM AT 23 C & 50% RH ( ASTM Standard d
882)
Breaking Breaking Modulus of Breaking Hc/Ha
Load elongation elasticity energy
Examples Mpa 'k Mpa KJ/m2
1 as produced 37.1 880 503 8600 0.44
1 washed 31.6 747 501 7750
1 cf. as 28.3 810 310 5640 0.07
produced
1 cf. Washed 20.0 120 603 327
2 as produced 31.2 880 520 8230 0.33
2 washed 25.8 637 631 6630
3 as produced 29.2 756 541 6194 0.29
3 washed 21.1 539 598 4930
4 as produced 24.5 662 632 5980 0.27
4 washed 20.2 521 606 4760
as produced 23.1 489 136 4155 0.07
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Table 2 shows the characteristics of roughness of the sheets
of examples 2-4, and comparison examples 1-2. A high level
of roughness, although spoiling the aesthetic appearance, is
critical for the printability of the sheet with printing
inks.
TABLE 2
SURFACE ROUGHNESS
EXAMPLE ROUGHNESS
(micron)
2 0.20
3 0.20
4 0.24
cf. 1 0.14
cf. 2 1.17
Tables 3 and 4 show test data for tear and impact traction.
TABLE 3
TEAR TESTS AT 23oC & 50% RH(*)
Examples Start tearing Propagation
N/mm N/mm
1 as produced 116.5 116.5
2 as produced 85.6 85.7
1 cf. as produced 64 63.8
(*) ASTM standard d-1938
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TABLE 4
IMPACT-TRACTION TESTS AT 10 C AND RH < 5% (**)
ON FILM OF 30 micron
Example Energy Load
KJ/m2 Mpa
1 110 30
2 73 24
1 cf. 6 12
2 cf. 22 23
145 18
(**)
The tests were carried out using instrumentation comprising a
conventional "piezoelectric load cell" for detecting the
energy, which cell is located on a terminal on which the end
of a test specimen is fixed, 30-40 micron thick, 30mm wide
and 35mm long.
A double incision was made symmetrically half way along the
sample such that each incision extended over a quarter of the
width of the sample. A rod was connected to the other end of
the sample, which rod acts as a guide for an axially-pierced
cylinder, 500g in weight. The rod terminates in a plate onto
which the weight is released from a height of 5cm at a
velocity of 1m/sec.
The apparatus was arranged within a climatic cell operating
at lOoC and RH < 5%.
The samples were conditioned at the same temperature for 48
hours before the test.
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TABLE 5
PROPERTIES OF SHEET FORMED VIA CAST-EXTRUSION
Examples Load Elongation Modulus
Mpa % Mpa
2 as produced 37.3 892 271
2 washed 30.1 630 464
3 as produced 35.0 846 379
3 washed 26.2 595 550
4 as produced 32.5 745 351
4 washed 21.0 531 495
Figures 1 and 2 show respectively the second derivative FTIR
and X-ray spectra of the composition of example 1.
